Moderate changes in the size of the outward (hyperpolarizing) current that is generated directly by the electrogenic Na/K exchange pump in the surface membrane of cardiac Purkinje fibers can cause substantial alterations in the shape of the action potential, in the level of the diastolic potential, or of the resting potential of quiescent cells, or in the rate of firing of spontaneously active preparations. Transient increments in Na/K pump current, of suitable magnitude, can be elicited experimentally in small canine Purkinje fibers by causing a transient increase in their intracellular Na concentration, [Na]i, and, thereby, a transient increase in the rate of electrogenic Na extrusion. Two techniques were used to increase [Na]i: in the first, the rate of Na extrusion from the cells was temporarily reduced by omitting K ions from the bathing fluid for short periods of time, in the second, the rate of Na entry into the cells was temporarily increased by electrically stimulating the preparations rapidly (e.g., greater than or equal to 2 Hz) for brief periods. After the extracellular K concentration was restored, or after electrical stimulation was stopped, respectively, use of a two-microelectrode voltage-clamp technique allowed the resulting increments in pump current to be measured directly, as changes in holding current. Increments in pump current elicited by these two methods in the same preparation decline with the same exponential time-course. In preparations stimulated electrically at a regular, low rate (e.g., less than or equal to 1 Hz) both methods of temporarily stimulating the Na/K pump cause a marked, transient reduction in the duration of the action potential. A closely similar reduction in action-potential duration to that observed during enhanced pump activity can be elicited by injecting, from an external source, a steady hyperpolarizing current of magnitude similar to that of the increment in pump current recorded in the same preparation under voltage clamp.